1. Field of the Invention
The present invention relates to a radome for protecting a radar antenna, for example.
2. Description of the Related Art
Generally, when a radar antenna is mounted to an aircraft, the antenna is placed inside a radome. When a radar antenna is mounted on a ship or on the ground, the antenna is also covered by a radome to protect against wind and subsequently smooth rotation of the antenna and to prevent reduction of electrical performance of the antenna due to adhesion of raindrops.
This kind of assembly is described in detail in xe2x80x9cRedoomu ni tsuitexe2x80x9d (xe2x80x9cRadome-Antenna Housingxe2x80x9d) by Takashi KITSUREGAWA, Mitsubishi Denki Gijutsu Hohkoku (Mitsubishi Electric Technical Reports), Vol. 29, No. 7, pp. 73-79, 1955.
FIGS. 12 and 13 are a perspective and a cross section, respectively, schematically showing a conventional radar assembly employing a radome.
In FIGS. 12 and 13, a radome 1 is called a half-wavelength plate radome, and is composed of a dielectric plate. A radar antenna 2 functioning as a radar device is disposed inside the radome 1. Reinforced plastics such as Fiber Reinforced Plastics (FRPs), polypropylene, or engineering plastics such as ABS resin, are used in the radome 1.
In consideration of the relative permittivity and dielectric dissipation factor of the dielectric material, this radome 1 is designed to permit passage of radio waves having a frequency used by the radar antenna 2 with minimal loss, in other words, reflection by the dielectric plate composing the radome 1 is reduced.
If we let xcex0 be the free space wavelength of the working radio wave, let ∈r be the relative permittivity of the dielectric material used, and let xcex8in be the angle of incidence of radio waves relative to the radome, then the thickness d of the dielectric plate composing the radome 1 is represented by Expression (1) below.
xe2x80x83d=(Nxcex0)/{2(∈rxe2x88x92sin2xcex8in)xc2xd}xe2x80x83xe2x80x83(1)
Moreover, N is a natural number, called the radome order.
Now, by making the radome 1 (dielectric plate) a thickness d which satisfies Expression (1), reflection by the radome 1 (dielectric plate) is reduced, permitting passage of radio waves having the frequency used by the radar antenna 2 with minimal loss.
The relationship between the radio wave frequency f, its free space wavelength xcex, and the speed of light c is given by Expression (2).
xcex=c/fxe2x80x83xe2x80x83(2)
Because a conventional radome 1 is constructed in the above manner, radio waves having a frequency which permits passage with minimal loss are constricted to radio waves having the working frequency of the radar antenna 2. Thus, one problem has been that when the radar antenna 2 is not being used, external radio waves having the same frequency as the working frequency of the radar antenna 2 also pass through with minimal loss, interfering with the radar antenna 2 and giving rise to malfunctions.
The present invention aims to solve the above problems and an object of the present invention is to provide a radome enabling interference in a radar device due to external radio waves to be reduced by enabling passage of radio waves having a frequency used by the radar device to be controlled and by preventing penetration by external radio waves having the same frequency as the radio waves used by the radar device when the radar device is not being used.
In order to achieve the above object, according to one aspect of the present invention, there is provided a radome which has a dielectric layer whose relative permittivity is changed by the application of an electric field, and an electric field applying means for applying the electric field to the dielectric layer.